1. Field of the Invention
This invention generally relates to Optical Transport Networks (OTNs) and, more particularly, to a system and method for scaling the total client date rate to match the available optical link capacity.
2. Description of the Related Art
FIG. 3 is a diagram depicting the G.709 OTUk/ODUk/OPUk frame format (prior art). The basic OTN container is an optical channel data unit (ODU) which can have a fixed rate (ODUk with k=0, 1, 2, 3, 4) or a variable rate (ODUflex). The G.709 ODUk frame format is identical for all rates (ITU-T G.709/Y.1331, “Interfaces for the Optical Transport Network (OTN)”, February 2012). It can be represented as a structure of four rows and 3824 columns. This structure, in turn, consists of various layers of signaling and overhead attached to a basic optical payload unit (OPU).
The OPU, spanning all four rows and columns 17-3824 is itself a composite of multiple tributary slots, each targeted for a rate of typically 1.25 gigabits per second (Gbps). Thus, the number of slots per frame is Ns=2, 8, 32 and 80 for k=1-4, achieving data rates of 2.5, 10, 40 and 80 Gbit/s respectively. By defining tributary slots, G.709 allows multiplexing of many lower-rate clients into an OTUk pipe. In this case, each client is mapped into this payload area through one of the constant bit rate mapping procedures, e.g. for SONET/SDH or Ethernet PCS, or through generic framing procedure (GFP), e.g. for MPLS packets or Ethernet MAC frames. Time division multiplexing of lower-rate ODUj into higher-rate ODUk (with j<k) is also supported. Each of the lower-rate ODUs is identified by a tributary port number and the appropriate amount of tributary slots is allocated to it.
FIG. 4 is a diagram depicting the position of a tributary slot within an OPUk payload area (prior art). The allocation of slots to sub-containers is signaled as part of the OPUk overhead, which constitutes columns 15 and 16 of the OTN frame structure.
FIG. 5 is a diagram depicting the multiplex structure identifier (MSI) signaled over 256 frames (prior art). Specifically, the multiplex structure is signaled end-to-end using the payload structure identifier (PSI) byte of each OTN frame (row 4, column 15). The signaling is accomplished over a multiframe, comprising 256 consecutive frames. Of the 256-byte PSI received in a multiframe, the MSI field starts at PSI byte 2 and has one byte entry per tributary slot. Each MSI entry indicates if the corresponding tributary slot is allocated to a tributary port or free.
In addition to the OPU payload and the OPUk overhead, the OTN frame also comprises ODUk overhead (rows 2-4, columns 1-14); OTUk overhead (row 1, column 1-14) which also includes frame alignment. In “standard” implementations of OTN, a Reed-Solomon based FEC is computed on the entire ODUk frame to obtain the last 256 columns of the OTUk frame.
FIG. 1 is a schematic diagram depicting a conventional OTN system (prior art). The OTN connects various sites over metro or long haul distances, as shown. The optical fibers running through the network carry many different physical channels, also known as “wavelengths”. A physical channel comprises a center wavelength typically in the optical C band, and a bandwidth (50 GHz with the standard ITU grid (ITU-T G.694.1, “Spectral grids for WDM applications”, February 2012) or higher in the case of “super channels”, see O. Gerstel et al, “Elastic Optical Networking: A New Dawn for the Optical Layer?” IEEE Comm. Magazine, February 2012. As shown in the figure, each site contains a reconfigurable optical add drop multiplexer (ROADM) which “drops” m receive channels {ARi} and “adds” n transmit channels {ATj}. Adjacent sites are connected by a stretch of optical fiber, divided into spans of typically around 100 km each. Each span is terminated using an optical amplifier, denoted R in the figure. These amplifiers, typically Erbium doped fiber amplifiers (EDFA), compensate for fiber attenuation. In addition, span termination may also include optical dispersion compensation.
FIG. 2 is a graph depicting optical signal-to-noise (OSNR) to the number of spans (prior art). A physical channel extends from the allocated transmitter site “T” to the allocated receiver site “R”, and may pass through ROADMs at many different intermediate sites. Amplifiers (both for span compensation and in EDFAs) add noise to the optical signal, hence the optical signal-to-noise ratio (OSNR) typically decreases with the distance spanned by the physical channel. The curve shows the available OSNR, with 12.5 dB OSNR being the required value for 100 gigabits per second (Gps).
The OSNR in a physical channel, along with other factors such as non-linearity, polarization mode dispersion, etc., determines the available capacity of the channel, i.e., the maximum spectral efficiency (b/s/Hz) that can be achieved on the channel.
The goal of communication system design is to choose modulation and forward error correction (FEC) schemes to achieve data rates close to channel capacity. The first step towards achieving this goal has been taken, with the use of coherent modulation at the transmitter and advanced signal processing at the receiver (see, for example, Savory et al., “Electronic Compensation of Chromatic Dispersion using a Digital Coherent Receiver”, Opt. Express, Vol. 15, No. 5, pp. 2120-2126, March 2007. Current deployments achieve a fixed rate of 100 Gbps over 50 GHz channels using polarization-multiplexed QPSK (ITU-T G.975.1, “Forward Error Correction for High Bit-rate DWDM Submarine Systems,” February 2004) (PM-QPSK), at a spectral efficiency of 2 b/s/Hz. Now, PM-QPSK requires an OSNR of around 12.5 dB to operate (more or less depending on the equalization and phase tracking algorithms and the forward error correction FEC used). As seen from FIG. 2, the available OSNR in the link is often greater than the minimum required. Limiting data rate to 100 Gbps in shorter links is clearly a waste of available capacity. To better utilize available channel capacity, next generation deployments target the use of higher order modulation schemes to achieve higher spectral efficiencies for links with high OSNR (see, for instance, D. L. McGhan, W. Leckie, C. Chen, Reconfigurable Coherent Transceivers for Optical Transmission Capacity and Reach Optimization, OW4C.7, OFC 2012).
It would be advantageous if the channel capacity of an OTN link could be more efficiently utilized.